Well service rigs can be used for a variety of tasks at oil and gas well sites, including, for example, swabbing, installing, and pulling a tube and sucker rod via large, chassis-mounted draw works. The well service rig can include a powertrain having an engine and a transmission that both provides propulsion to the vehicle drive line as well as driving the winches of the draw works. Downstream of the transmission, an output transfer gear group, or drop box, can be provided to couple and decouple the two drive lines from the engine-transmission power module. Existing drop boxes have been controlled by manually operating couplers to couple/decouple the drive lines. Control of the drop box can be difficult, leading to situations where the drop box and the transmission can be operated inefficiently and/or damaged.